COMMUNITY SUCCESSION AND DEVELOPMENT 



567 



bury and Alden (1899) and more recently 

 by Fryxell (1927); the general picture of 

 succession was developed by Cowles ( 1899, 

 1901, 1911) with particular reference to 

 species of plants, and for plant and animal 

 populations by Shelford (1913); the vege- 

 tation was reported upon by Fuller (1925); 

 the annual and seasonal march of daylight 

 intensities in the open sun, clearings, and 

 canopy shade of several serai stages has 

 been measured (O, Park, 1931), as has 

 the ultraviolet component (Strohecker, 

 1938), as well as the soil moisture, growtli- 

 water, and rates and amounts of evapora- 

 tion (Fuller, 1911, 1914). In addition, sev- 

 eral groups of animals have been examined 

 in terms of community distribution and 

 tolerances, for the several community factor 



In addition to polyvalent, tolerant species 

 that range widely, possibly through an 

 entire sere, each serai stage has its typical 

 or characteristic organisms. Such species 

 are biotic indicators, or indices of succes- 

 sion. They may be restricted to a particular 

 serai stage, as a result of narrow limits of 

 toleration or pecuhar habitat requirements; 

 or they may have a broad distribution 

 through several serai stages, but reach a 

 relatively high frequency of abundance in 

 a particular type of community. 



Such a sere has both physiographic and 

 biotic (developmental) influences at work, 

 interacting within each type of community, 

 and shaping its normal destiny (Fig. 206). 



It should be remembered that within 

 each relatively independent major commu- 



Fig. 204. Progressive decrease in rate of evaporation from pioneer to climax community in 

 the forest-on-sand sere of northern Indiana. Measurements are mean average daily evaporation 

 rales in centimeters from a standard atmometer during the 10 midsummer weeks for the years 

 1910, 1911, and 1912. (Modified from Fuller.) 



complexes— for example, beetles (O, Park, 

 1930, 1931a), orthopterans (Strohecker, 

 1937), ants (Talbot, 1934), and spiders 

 (Lowrie, 1942). 



Generally speaking, this hterature in- 

 dicates that as one walked inland, starting 

 at the Lake Michigan beach and ending 

 with the maple-beech climax, the several 

 serai stages, in the period of foliation, would 

 show a progressive decrease in total light 

 intensity (Fig. 183), in ultraviolet (Fig. 

 203), in wind velocity, and in the rate of 

 evaporation (Fig. 204). In correlation with 

 these decreases, there would be a progres- 

 sive increase in the amount of soil moisture, 

 in the relative humidity, and the amount of 

 humus in the subterranean stratum, the 

 amount of leaf and log mold on the forest 

 floor, and in the number of micro-arthro- 

 pods per kilogram of floor mold. 



Each serai stage, then, has a characteris- 

 tic physical environment (Fig. 205) and a 

 characteristic biota. 



nity, many minor successional processes are 

 being carried on. These take place at differ- 

 ent rates, and often have a highly character- 

 istic sequence of serai stages. As a general 

 principle applicable to major communities, 

 these intracommunity successions in habitats 

 or minor communities are completed before 

 the whole community is replaced; in fact, 

 many such minor successions are partially 

 responsible for the developmental changes 

 that take place within the larger whole. 



Thus within the grassland communities 

 there is a well-defined sequence of stages in 

 the aging of the dung of animals. This 

 sequence has been investigated by Mohr 

 (1943) for cattle droppings in open 

 pastures in central Illinois. The physical 

 changes that take place are generally as- 

 sociated with progressive loss of water 

 content, and are given as: (1) a pioneer 

 stage, in which the freshly deposited dung 

 is greenish brown with distinct tarnish and 

 film, (2) uniformly brown and moist, (3) 



